1. Field of the Invention
The present invention is related to a magnetoresistance device and method of fabricating the same, particularly, to a magnetoresistance device including a magnetic tunnel junction (MTJ), which exhibits a tunneling magnetoresistance effect (TMR), and method of fabricating the same.
2. Description of the Related Art
A resistance of a magnetic tunnel junction, which consists of two ferromagnetic layers and a tunnel barrier layer (or a tunnel dielectric layer) disposed therebetween, changes depending on the relative direction of the magnetizations of the magnetic tunnel junction. Such phenomenon is known as a tunnel magnetoresistance effect. Measuring the resistance of the magnetic tunnel junction allows the detection of the direction of the ferromagnetic layers. Making use of such characteristics of the magnetic tunnel junction, magnetoresistance devices including magnetic tunnel junctions are used for magnetic random access memories (MRAMs), which store data in a non-volatile fashion, and magnetic read heads for hard disc drives.
Magnetoresistance elements, which include a magnetic tunnel junction, are typically composed of a fixed magnetic layer, a free ferromagnetic layer, and a tunnel dielectric layer disposed between the fixed and free magnetic layers. The fixed magnetic layer has spontaneous magnetization whose direction is fixed, while the free magnetic layer has spontaneous magnetization whose direction is reversible.
In order to tightly fix spontaneous magnetization, fixed ferromagnetic layers are often formed to be in contact with antiferromagnetic layers. Exchange interaction provided by an antiferromagnetic layer tightly fixes the spontaneous magnetization of a fixed ferromagnetic layer. In general, antiferromagnetic layers are formed of manganese-including antiferromagnetic materials, such as IrMn, and PtMn.
Furthermore, a free ferromagnetic layer is often composed of hard and soft ferromagnetic layers, the hard ferromagnetic layer being made of ferromagnetic materials having high spin polarization ratios, and the soft magnetic layer being made of soft ferromagnetic materials. Such structure of a free ferromagnetic layer reduces the coercive force of the spontaneous magnetization of the free ferromagnetic layer with an increased magnetoresistance ratio (MR ratio) of the magnetic tunnel junction. Hard ferromagnetic layers are generally made of cobalt-including ferromagnetic materials, such as Co and CoFe, while soft ferromagnetic layers are generally made of nickel-including ferromagnetic materials, such as NiFe.
One of the issues of magnetoresistance elements is thermal stability. Subjecting magnetoresistance elements to a high temperature causes interdiffusion between layers incorporated therein. This interdiffusion deteriorates the characteristics of the magnetoresistance elements, especially the magnetoresistance ratio. Patent document 1 presents a problem concerning interdiffusion between hard and soft ferromagnetic layers. The essential point is that nickel included in the soft ferromagnetic layer is diffused into the hard ferromagnetic layer. Diffusion of nickel into a hard ferromagnetic layer deteriorates the magnetoresistance element. Japanese Open Laid Patent Application No. P2000-20922A discloses that an oxide or nitride layer for preventing interdiffusion is disposed between hard and soft ferromagnetic layers. Japanese Open Laid Patent Application No. Jp-A 2002-158381 addresses a problem that manganese from manganese-including antiferromagnetics diffuses into fixed ferromagnetic layers. This documents discloses a technique for avoiding diffusion of manganese into a fixed ferromagnetic layer by incorporating two ferromagnetic layers and a dielectric or amorphous layer disposed therebetween into the fixed magnetic layer. Japanese Open Laid Patent Application No. Jp-A 2001-237471 discloses a technique for improving thermal stability of magnetoresistance elements by inserting magnetic oxide layers into fixed and free ferromagnetic layers. Japanese Open Laid Patent Application No. Jp-A-Heisei 10-65232 addresses a problem of interdiffusion between a free ferromagnetic layer and a buffer layer disposed under the free ferromagnetic layer. This document discloses a technique for reducing the thermally induced diffusion into the buffer layer and thereby improving thermal stability by disposing an atomic diffusion barrier layer formed of oxides, nitrides, carbides, borides, or fluorides between the free ferromagnetic layer and the buffer layer.
Japanese Open Laid Patent Application No. Jp-A 2002-74627 discloses a technique for increasing electron reflectivity and thereby improving thermal stability by disposing a high conductivity layer and an electron reflection layer which is substantially crystalline, and mainly includes an element different from the main element used for the high conductivity layer. This document also discloses that the electron reflection layer includes a first layer close to the free magnetic layer, and a second layer far from the free magnetic layer, the first layer consisting of an oxide of an element that is more easily oxidized than that consisting of the second layer.
Another issue of magnetoresistance elements is reduction in coercive forces of free ferromagnetic layers. A free ferromagnetic layer composed of a layered structure including hard and soft ferromagnetic layers may not have the coercive force reduced sufficiently in the case that an increased MR ratio is required.
Reduction of the coercive force can be achieved by reducing the product of magnetization Ms and thickness t of the free ferromagnetic layer (which is referred to as “the product Ms·t”, hereinafter). An explanation of the reason is given in the following. For a uniaxial free ferromagnetic layer, the coercive field thereof depends on a total anisotropy field of the free ferromagnetic layer. For the magnetoresistance element having the size of the sub-micron order, a total anisotropy field of the free ferromagnetic layer mostly arises from the shape-induced anisotropy field of the free ferromagnetic layer. Therefore, the coercive force of the ferromagnetic layer is approximately equal to the shape-induced anisotropy field Ha. For this case, the anisotropy field Ha is represented by the following equation (1):Ha=4πMs(Nx−Ny),  (1)where Ms is the saturated magnetization of the free ferromagnetic layer, and Nx and Ny are demagnetization factors of the magnetoresistance element in the directions along the long and short sides, respectively. For a given thickness of the free ferromagnetic layer, Nx−Ny increases as an increase in the ratio of the long side to the short side (or the aspect ratio), and this results in an increase in the shape-induced anisotropy field Ha. Furthermore, the reduction in the size of the free ferromagnetic layer increases the anisotropy field Ha, because of the increase in the demagnetization factors as the reduction in the size of the free ferromagnetic layer. For a fixed aspect ratio, the anisotropy field Ha is approximately described by the following equation (2):Ha≅4πMs·t/W,  (2)where W is the length of the short side of the free ferromagnetic layer, and t is the thickness of the free ferromagnetic layer. The equation (2) indicates that the coercive force of the free ferromagnetic layer can be reduced by reducing the product Ms·t. In general, reducing the thickness t of the free ferromagnetic layer achieves the reduction in product Ms·t, and thereby reduces the coercive force of the free ferromagnetic layer.
Additionally, Zhang et al. discloses a magnetic tunnel junction including an FeOx layer disposed between an Al2O3 layer and a CoFe layer, in Applied Physics Letters vol. 89, No. 19, 7 May 2001, pp. 2911-2913.
Furthermore, Matsuda et al. discloses that magnetoresistances of magnetic tunnel junctions are reduced by a geometrical structure of the junctions in Applied Physics Letters, vol. 77, No. 19, 6 Nov. 2000, pp. 3060-3062.
In addition, Moodera et al. discloses magnetoresistances of magnetic tunnel junctions are increased by a geometrical structure of the junctions in Applied Physics Letters, vol. 69, No. 5, 29 Jul. 1996, pp. 708-710.
Also, Ohnuma et al. discloses a technique for forming a highly resistive soft magnetic film with granular metal consisting of cobalt base alloy, iron base alloy, and a non-magnetic oxide or nitride.
The inventor of the present invention has discovered that interdiffusion between a magnetoresistance element and a conductor electrically connecting the magnetoresistance element to other elements causes an undesirable influence on the characteristics of the magnetoresistance element. Operating a magnetoresistance element requires electrical connections of the magnetoresistance element with other elements (such as transistors). Therefore, a magnetoresistance element is connected to conductors such as via contacts and interconnections that are electrically connected to other elements. In general, such conductors are formed of aluminum, copper, tungsten, or titan nitride, as is the case of other semiconductor integrated circuits. Tantalum, ruthenium, zirconium, or molybdenum may be used for a conductor providing electrical connections between magnetoresistance elements and other elements. The interdiffusion between a magnetoresistance element and a conductor that electrically connects the magnetoresistance element to other elements causes the following three influences.
Firstly, the diffusion of material included in the conductor into the magnetic tunnel junction reduces the MR ratio thereof. For a conductor including aluminum, diffusion of aluminum into the magnetic tunnel junction is especially serious, because aluminum is diffused by applying relatively low temperature. In another aspect, the diffusion of the material of the conductor is essential because the diffusion of material included in the conductor, especially aluminum, into the antiferromagnetic layer and the soft ferromagnetic layer promotes the diffusion of manganese from the antiferromagnetic layer and nickel from the soft ferromagnetic layer into the tunnel barrier layer.
Secondly, the diffusion of material included in the magnetoresistance element into the conductor, which electrically connects the magnetoresistance element with other elements, increases the resistance of the conductor. The increase in the resistance of the conductor deteriorates the SN ratio for detecting the resistance of the magnetic tunnel junction. Especially, since manganese included in the antiferromagnetic layer, and nickel in the soft ferromagnetic layer are diffused by relatively low temperature, the increase in the resistance caused by the diffusion of manganese and nickel is of significance.
Thirdly, thermal diffusion of material of the free ferromagnetic layer into the conductor that electrically connects the free ferromagnetic layer with other elements makes it difficult to achieve reduction in the coercive force through reducing the thickness t of the free ferromagnetic layer. This is because diffusion caused by a thermal treatment causes a large change in the characteristics thereof, and thus reduces operation reliability of the magnetoresistance element when the free ferromagnetic layer is decreased in the thickness. FIG. 26 is a graph illustrating influences caused by thermal treatments on 4πMs·t of free ferromagnetic layers having reduced thicknesses t. The characteristics of the free ferromagnetic layers are obtained under the condition described in the following; the structure of the samples is sub./Ta(10 nm)/AlOx/Ni81Fe19/Ta(10 nm). The AlOx films are formed through oxidizing aluminum films having a thickness of 1.5 nm. The thicknesses of the Ni81Fe19 films are selected out of the values of 3.0 nm, 2.6 nm, and 2.2 nm. The Ni81Fe19 films deposited through sputtering. The temperature of the thermal treatment ranges between 250° and 400° C., and the duration is 30 minutes. Magnetizations Ms are measured with a vibration magnetometer.
As illustrated in FIG. 26, thermal treatment on the Ni81Fe19 films having thicknesses t less than 30 nm causes drastic changes in 4πMs·t thereof; furthermore 4πMs·t is remarkably decreased as the decrease in the thickness t and the increase of the temperature of the thermal treatment. These samples exhibit poor repeatability. As thus-described, thermal treatment destabilizes 4πMs·t of the free ferromagnetic layer when the thickness thereof is reduced down to 3 nm. Such instability prevents reduction of the thickness t of free ferromagnetic layers.
There is a need for providing a technique for reducing interdiffusion between a magnetoresistance element and a conductor providing electrical connections between the magnetoresistance element and other elements.